The present invention relates to centrifugal separation apparatus for separating particulate contaminants from liquids, such as engine lubricants, passed therethrough to effect cleaning, and in particular relates to rotor means used within such apparatus to perform the actual separation and retention of such contaminants.
Centrifugal separation apparatus is well known for use within the lubrication systems of vehicle internal combustion engines as efficient means for removing very small particulate contaminants from the constantly recirculated liquid lubricant over a long period of operation, such particulate contaminants arising from abrasion of the metallic components of the engine, decomposition of the lubricant and products of combustion.
Such centrifugal separation apparatus is sometimes known as being of the sedimenting, solid-wall type in which separated solids are retained within the rotor means as a sediment against an impervious radially outer side wall thereof, and distinct from the so-called filtering perforate-wall type in which the solids are held by the mesh of a perforate radially outer side wall while liquid passes therethrough.
Insofar as such separators are responsible for cleaning a liquid which is in any event circulated at elevated pressure, the art has concentrated on employing such lubricant pressure to effect rotation of parts responsible for generating centrifugal forces, and as such it includes rotor means comprising an essentially closed vessel, or canister, being supported for rotation about a rotation axis within a housing, and supplied with the liquid lubricant at elevated pressure at the axis. The canister is filled with the liquid and assumes a significant internal pressure before liquid is forced from the base (or other peripheral wall) of the canister by way of tangentially directed jet reaction nozzles, the reaction to said ejection causing the rotor canister and liquid within it to spin at high speed about the axis and thereby force solid particles to migrate from the liquid passing through the canister and agglomerate into a cohesive mass on the peripheral walls spaced from the rotation axis. The reaction nozzles, being directed substantially tangentially with respect to the rotation axis, at least in a plane orthogonal to the axis, define a reaction turbine.
It will be appreciated that the efficiency of separation is inter alia dependant upon creating the conditions in which any liquid entrained particle can migrate radially to the nearest deposition surface and is a function of the force acting on such particle and the time for which it can act. The former is a function of rotation rate and distance from the rotation axis. The latter is a function of the time taken for the entraining liquid to pass through the rotor canister (also called the residence time) and the proximity of the deposition surface, and may be considered in terms of an effective residence time, that is, influencing the contribution of the actual residence time by positioning the contaminated liquid relatively to an appropriate deposition surface. However both the rotation speed of the rotor canister and contained liquid, and the rate at which liquid is passed through and ejected therefrom, are dependant upon the pressure drop between the canister contents and housing and upon the dimensions of the nozzles, within the constraints of such nozzle dimensions providing sufficient torque from the turbine to overcome inertial and frictional resistance to commencement of, and continuation of, rotation.
Within an internal combustion engine where lubricant is circulated under an initial (pumped) pressure in a range of about 2 to 6 bars that varies with operating conditions, a canister of relatively modest diameter, say 10 to 15 cms, and reaction turbine nozzles may achieve a rotation speed in the range of 4000 to 9000 r.p.m. which is sufficient for removing the relatively dense, contaminants of lubricant residue and metallic particles traditionally considered to be of principal detriment to the engine.
Examples of such reaction turbine centrifugal separation are shown in GB 745377, GB 2328891, U.S. Pat. No. 5,575,912 and U.S. Pat. No. 5,906,733, and it can be seen that as developments have been made to increase efficiency of separation, and range of separability, the degree of structural complexity has also increased, not least in optimizing effective residence time and/or placing the liquid to maximize forces acting upon entrained contaminants for the limited rotation forces available.
This is particularly true in respect of the dual goals of deriving maximum rotation energy from the liquid passing through the rotor whilst providing therein conditions necessary and suited to centrifugal separation of low density contaminant particles such as soot. Such contaminants are now seen as an important cause of engine wear, particularly in compression ignition engines, and require the lubricant to be provided with greater effective residence time and/or be subjected to greater centrifugal forces than hitherto, notwithstanding that providing such conditions in these arrangements also tend to militate against efficient flow of liquid through the canister.
Obtaining greater rotation rate from such a reaction turbine necessitates ejecting liquid at a greater rate, by increasing the pressure and/or by shortening the residence time or by increasing the volume of liquid contained, whereas attempting to cause the contaminant entraining liquid to traverse the canister at a greater radial distance from the axis is made difficult by the fact that the rotating liquid content of the canister creates a radial pressure gradient tendering to keep newly introduced liquid away from the radially outer region of maximum centrifugal force (unless internal structures are provided that add to the complexity and/or consume energy from the rotation). Therefore, optimizing such rotor canister is not a matter of simply increasing the radial dimensions of the canister but effecting a compromise that nevertheless includes containing within the canister at high pressure a relatively large volume of the liquid lubricant to enable it to have a significant effective residence time while it follows a tortuous path that involves interchanging potential and kinetic energy until it is ejected with sufficient energy for rotation production.
U.S. Pat. No. 6,017,300 in particular explains in some detail that for properly separating very lightweight soot particles that can contaminate the liquid lubricant as products of combustion, the particles have to be subjected to higher centrifugal forces than readily available from such traditional, reaction turbine drive centrifugal separation arrangements, along with a longer residence time, and proposes to elaborate upon the complex cone stack arrangement of U.S. Pat. No. 5,575,912 by an external impulse turbine, the latter providing for high rotation operation and, being separate from the liquid for cleaning in the container, permits the contaminated liquid to have a longer residence time.
Separating low density contaminants from constant streams of high pressure liquid is not the only situation for which traditional centrifugal separator designs are inadequate. For example, as described in U.S. Pat. No. 5,906,733 where the liquid to be cleaned is derived only indirectly from a high pressure circulation, either at low pressure or intermittently, a separate flow of the high pressure liquid is employed to effect rotation of the canister whilst the liquid to be cleaned can flow through at lower pressure and/or at lower rate, the separate flow of liquid effecting rotation by way of direct reaction jet nozzles in the container or as an impulse turbine employing external blades against which liquid is directed from stationary nozzles.
Insofar as these modified designs still adopt the principle of defining a rotor vessel whose radial dimensions are optimized for centrifugal forces on liquid entrained particles and function by filling it with the contaminated liquid and then effecting rotation at appropriate speed, they still exhibit significant rotor vessel inertia and have to provide energy to overcome frictional and other bosses, providing a slow response, particularly in start-stop situations.